That's a tall plant, #1, and looks likely to be as you said, but I'm not great at distinguishing the karwinskiana/voburnensis subspecies.

As for the 2nd.....what a great plant, only solution is something like a dustbin lid!

....and a touch of weedkiller for the oxalis. I use a glyphosate solution, and just paint the top leaves with an art paint brush. Doesn't seem to affect the main plant, though I was worried about it leaching out of the dying plant, but its supposed to be rendered ineffective when it gets to soil.

Yes the oxalis can be difficult to eradicate. I have a large Notocactus magnifica with some oxalis in it but a different species - the bulbous kind. I have painted glyphosate on it at least 6 times and each time it disappears for a while only to pop up again later!I know a member of the local CAS society here who sprays the weeds among his succulents with glyphosate - weeds, succulents and all - and told me the herbicide does not affect the plants as their waxy cuticle prevents any penetration. I tried this on a bed of Senecio and it seems to have worked. No apparent damage to the plants. Not sure if I would do that on valuable plants though!